This invention relates to methods and apparatus for high frequency welding of thermo-adhesive sheets and the product produced thereby, and more particularly to electrode dies for use in a high frequency welding press for producing a reinforced pocket structure when sealing superposed thermoadhesive sheets and to the product having the reinforced pocket structure so produced.
The invention finds particular application during the fabrication of photo finisher wallets or pouches. Most non-professional photographers do not themselves process their exposed camera film or produce prints therefrom, but instead entrust these activities to a photo finishing laboratory. Typically, the customer delivers his exposed film to a photo finisher laboratory which in due course returns to the customer a package containing the finished photographic prints and the corresponding negatives. Such a package typically includes a wallet or pouch containing a stack of the finished photographic prints and another stack of the corresponding negatives usually as film strips within a protective sleeve.
One popular type of photo finisher wallet or pouch is constructed of a flexible sheet material such as polyvinyl chloride (or vinyl), which is foldable along a middle portion to form a folder having a front cover and a rear cover. Each cover is larger in each dimension than the corresponding dimensions of the photographic prints, and one of the covers includes a pocket on its inner surface into which the stack of prints may be inserted. The sleeved film strips may be inserted in the same pocket, or a separate pocket for accommodating the film strips may be on the inner surface of the other cover, or the film strip pocket may be situated upon the print pocket. In any case, each pocket is formed by a panel of flexible material affixed along three of its edges to the cover's inner surface, normally with the panel's unaffixed edge or pocket opening parallel to and directed toward the folder's middle portion.
One method of fabricating such photo finisher wallets is by utilizing flexible sheets of thermo-adhesive plastic materials, such as vinyl, polypropylene or polyethylene, in superposed relation, and to seal the edges of the superposed sheets where required to form the cover and the pockets. Specifically, a high frequency (or radio frequency) welding press is utilized, such apparatus including a horizontal lower electrode platen and a vertically movable upper electrode die, for producing an inductive field between the die and the platen with the superposed thermo-adhesive sheets interposed therebetween. The upper die includes a seam producing die surface for producing a welded seam sealing the superposed sheets when pressure is applied thereto by the upper die in the presence of the inductive field. The resulting welded seam extends along at least three edges of the cover sheet, and along three edges of the other sheet leaving an unseamed edge, sealing the sheets along the seam edges to form a pocket having an opening.
High frequency pressure welding apparatus and methods for sealing thermo-adhesive plastic sheets to one another are well known; see, for example, U.S. Pat. No. 2,638,963 issued to E.R. Frederick et al. and U.S. Pat. No. 4,043,858 issued to E. Dantowitz.
With respect to the photo finisher wallet, it can be appreciated that the junction of the unsealed pocket edge with the seam at either end of the unsealed edge (i.e. each corner of the pocket at the pocket opening) is subject to stress by forces exerted on the pocket by the contained stack of photo prints, as well as by forces exerted on the pocket incidental to each insertion and removal of the stack of prints and the stack of film strips. Such stresses may result in tearing of the pocket sheet at the pocket opening along the intersecting weld seam.
One manner of increasing the pocket's resistance to tearing under normal stresses involves producing a reinforcing weld at each corner of the pocket at the pocket opening. The reinforcing welds are formed at the same time the welded seam is produced for sealing the superposed sheets to one another, with the seams and the reinforcing welds formed by the upper electrode die having widened die surface portions inwardly extending from the seam producing die surface, the widened surface portions overlying respective corners of the pockets at their respective openings during the welding process.